Non-contact voltmeter increases safety and speed

Berkeley, CA--Poking around in an electrical junction box with test probes sometimes feels like disarming a live bomb. One slip and 120 volts of 60-cycle current-or worse-will shoot up your arm like a lightning strike. Sobering government statistics show that 7% of workers killed in industrial accidents die of electrocution.

Yet, to measure voltage or power, technicians must touch their multimeter probes to bare, stripped wires or terminals. Ordinary non-contact, clamp-on devices only measure amperage, not voltage. Or at least they used to.

Engineers at Electrascan say their VIP500 is the first and only multimeter able to measure voltage, power, and power factor through insulated wires. Intended for 50/60-Hz or 380/420-Hz alternating current, the device saves time, money, and possibly lives. It also can take voltage readings in many situations where conventional meters cannot.

The unit consists of a pair of rather ordinary-looking clamp-on probes and a multimeter module with digital LCD readout and function-selection knob. To take a measurement, the user simply clips the probes to the desired wires and reads the LCD. Voltage reads to an accuracy of 2%+1V for wires ranging in diameter from ¼ inch to ½ inch and within 4% for wires smaller than ¼ inch. Amperage registers within 2% from 0 to 200A and power within 5% from 0 to 50 kW. Intended for field use, the hand-held VIP500 operates for a minimum of 20 hours on four AA batteries.

The meter's voltage-measurement concept builds on the basic capacitive technology found in the ubiquitous ignition-timing analyzers used by auto mechanics. "But they merely give a crude picture of the waveform, not absolute voltage values," says Joel Libove, vice president R&D at Electrascan and co-inventor of the VIP500.

Capacitive voltage devices "only provide a crude picture of the waveform" because-prior to the VIP500-no one thought them capable of anything more. Capacitance varies enormously with changes in wire thickness, geometry, insulation type and thickness, the presence of the probe's clamps, and other often-unknown conditions. To make accurate non-contact voltage measurements, a meter would have be able to find the capacitance of any wire/probe combination presented to it. Only then could it adjust the voltage readout to account for the measured capacitance. And that's just what the VIP500 does.

To find a wire's capacitance, the voltmeter induces a known 6-kHz reference voltage in the wire via the clamp-on probe's two sensing-plate assemblies. It then observes the measured value of this induced voltage and-because it knows what the value should be-adjusts the meter's gain until the 6-kHz voltage reads at the expected level. This process automatically corrects the VIP500 across a broad range of frequencies-up to several megahertz-by accounting for the capacitance of the particular wire measured.

Engineers chose 6 kHz to avoid significant harmonics at the most commonly measured frequency, 50/60 Hz. "Higher frequencies would work even better," explains Libove, "but they would demand more power and shorten battery life." The dynamic compensation system continuously adjusts the meter to account for reasonable fluctuations in the wire's capacitance.

Libove and partner Jay Singer, Electrascan's president, stress that while their invention appears to be a simple ammeter spinoff, the VIP500 is quite a different animal, consuming more than three years of engineering work. "It's a deceptively complex device," says Libove. "There are several sophisticated subsystems, for instance, that correct errors. And making it read the correct value for wires ranging from lamp cord to something ¾-inch thick-a capacitance range of possibly six to one-took quite some time."

Though safety heads the VIP500's benefit lists, Singer and Libove believe that speed, convenience, and cost savings will ultimately drive the unit's acceptance with customers. They've already identified several niche applications for which no other voltmeter will work. "With aircraft wiring, it's sometimes impossible to get to a junction box," says Singer, elaborating on one application. "You can't break the insulation to take conventional measurements because splices in the wires aren't permitted for safety reasons."

Inspiration for the device grew from Libove's frustration with taking voltage measurements during a project three years ago. Singer kiddingly challenged him to invent a non-contact method. And, after the two debated some possible solutions, they stayed up all night creating a proof-of-concept prototype. "We were driven by the desire to measure something that could not be measured before," says Libove.

Other Applications

Laboratory instruments

Any device requiring passive voltage measurement

Sometime this year Electrascan hopes to announce a model that measures both ac and dc voltage. Libove considers this an even bigger challenge than the VIP500. Fortunately, Electrascan's pending patents cover a variety of non-contact voltage-measurement techniques. Says Libove: "This first product uses a method that keeps costs down, yet still achieves good precision with ordinary components."

Joel Libove received his doctorate in electrical engineering from the University of California, Berkeley, under the tutelage of Jay Singer, who is now his business partner. An inventor and entrepreneur, he has founded companies that produce computer software and hardware, and electrical test/measurement equipment. Libove holds patents for hard disk controllers, nuclear-magnetic-resonance imaging, and a dry-electrode electroencephalograph.

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